1. Field of the Invention
The invention is related to the field of optical communication networks, and in particular, to methods and networks for optimizing Polarization Mode Dispersion (PMD) measurements based on temperature for installed fibers.
2. Statement of the Problem
Many communication companies use an optical network for transmitting data because of its high-bandwidth capacity. Fiber optic cables in the optical network reliably transport optical signals over long distances between a transmitter and a receiver. The fiber optic cables are comprised of transmission fiber, such as a single mode fiber (SMF). Over the length of SMF, the optical signals experience some degradation due to attenuation along the fiber. Fiber background loss in the fiber causes the attenuation, typically about 0.2 dB/km. The optical signals also degrade due to other limitations, such as Polarization Mode Dispersion (PMD), chromatic dispersion, and nonlinear effects.
PMD is a dynamic pulse broadening phenomena. In a single mode fiber, optical pulses propagating down the fiber may separate into two orthogonal modes of polarization that travel at different speeds. The relative amplitudes of these two pulses are determined by the state of polarization of the input pulse relative to the fiber's input principal states of polarization (PSP). The separation into the two orthogonal modes may be caused by intrinsic and extrinsic factors. The intrinsic factors may result from fiber manufacturing processes, such as core ellipticity, or built-in asymmetric stresses. The extrinsic factors may be caused by stresses due to twisting, bending, and environmental effects, such as temperature and thermal gradients.
If the core of the fiber has a perfectly circular cross-section, then both modes travel at the same speed over the same distance. Otherwise, one mode travels slower than the other resulting in a difference in group velocities (an effect called birefringence). The difference in velocities between polarization modes is wavelength dependent and time dependent. The difference in propagation time, Δτ, experienced by the two polarization modes at a given wavelength is referred to as the differential group delay (DGD) with units in picoseconds (ps). When the DGD in a fiber becomes excessively large, a receiver is unable to distinguish between a zero bit and a one bit, and bit errors occur eventually resulting in a PMD-induced outage.
Network engineers test installed or buried fibers to determine the effects of PMD on the fibers. To test a fiber, two network engineers are needed. One network engineer travels to an access point on the fiber and connects a light source for the PMD test onto the fiber. The other network engineer travels to another access point, possibly several kilometers away, and connects a PMD test unit onto the fiber. The access points may be a POP site, a manhole, a splice point, etc. With the equipment attached to the fiber, the light source transmits light over the fiber and the PMD test unit measures PMD on the fiber. The network engineers commonly take two PMD measurements for the fiber. Because fiber optic cables typically include many fibers, the network engineers may perform the same test for each fiber of the fiber cable.
Unfortunately, due to the stochastic nature of PMD on fibers, the tests by the network engineers may not accurately reflect the PMD on the fiber at any given time. For instance, the two measurements taken by the network engineers may reflect a low PMD for the fiber, when the maximum PMD may far exceed the measured PMD. The network engineers would have to take measurements over a long period of time to get an accurate measurement of the PMD, which is inefficient and impractical for the PMD tests and is not cost effective.
A major factor contributing to the fluctuation of PMD is temperature. Temperature variations can cause induced stress on a fiber. The stress may cause PMD to fluctuate rapidly or slowly on the fiber. Thus, the accuracy of the PMD measurements depends on the ambient temperature of the fiber. One problem with current PMD tests on installed fibers is that the timing of the PMD measurements is not correlated with temperature to provide more accurate measurements.